1. Field of the Invention
The invention relates to an ultrasonic device including an ultrasonic transducer and a generator for generating pulses for the transducer, the generator being connected to the transducer via an active impedance matching device. The invention can be used notably in the field of non-destructive testing by means of ultrasound.
2. Description of the Related Art
A current trend in notably the cited technical field is the use of ultrasonic transducers having a very large bandwidth and a short pulse duration and offering an excellent spatial resolution. Their use, however, is impeded by the difficulty in matching their electric impedance to that of the connection cable, generally speaking a coaxial cable having an impedance of 50 ohms, throughout their passband (this restriction is particularly annoying for the testing of large objects for which long cable lengths are required). Actually, at the end of its connection cable for connection to the generator for generating the excitation pulse signals, the transducer which receives the electric signals transmitted by said generator and corresponding to the ultrasonic signals to be transmitted in the ultrasound propagation medium is only rarely matched, because the impedance of a transducer is a complex variable which is difficult to define, notably its 50-ohm matching being limited to a single frequency, i.e. the central frequency for which the transducer has been manufactured. Moreover, even when realised approximately, matching is accompanied by a disturbing inductive effect in the form of a counter-electromotive force. Finally, even in the receiving mode the excitation pulse signal generator does not "see" a 50 ohm impedance, except again for a single frequency.
French Patent Application No. 2501447 describes an assembly for exciting an electroacoustic transducer by means of an electronic generator which utilizes locking means for maintaining a maximum transfer of energy between said generator and the transducer, which means measure the dephasing between the intensity of the current and the voltage supplied by the generator and correct the reactive component of the impedance of the transducer in a sense appropriate to eliminate said dephasing. Such impedance matching enables the operating frequency band of the transducer to be enlarged to a given extent, but it remains comparatively inaccurate since the means used are based on inductive elements and are hence sensitive to the experiment conditions, notably to the length of the connection cables between the generator and the transducer.